1. Field of the Invention
The present invention relates to an acceleration sensor, and is applicable to an automobile air bag system or other consumer devices.
2. Description of the Background Art
Today, many automobiles employ an air bag system, into which an acceleration sensor for detecting an impact is generally incorporated.
Efforts have been made on the acceleration sensor to be smaller and less costly so that the acceleration sensor may be incorporated into various automobiles. For example, as a material for a package covering a semiconductor chip which constitutes an acceleration detector and a signal processor of the acceleration sensor, metal has been replaced by resin.
A mass body, as a movable part for detecting acceleration, is provided on a substrate surface of a semiconductor chip including the acceleration detector. To provide movable space of the mass body, and to prevent dirt or water from entering into the movable space, a cap is bonded to the substrate surface of the semiconductor chip, thereby hermetically sealing the movable space. Such technique is disclosed in Japanese Patent Application Laid-Open No. 8-114622 (1996). Other documents related to the present application include Japanese Patent Application Laid-Open Nos. 10-2911 (1998); 2001-185737; and 2001-337105.
As described, in the acceleration sensor, a cap is provided on the surface of the substrate for the semiconductor chip. This cap is likely to receive stress exerted thereon from a resin package, which is caused by the change in ambient temperature during molding of the resin package, or when the acceleration sensor is being used.
When stress is exerted on the cap, while depending on the strength and frequency of the stress, a crack may occur in the cap. The crack thereby generated is likely to break hermetic sealing of the movable space of the mass body.
When hermetic sealing is broken, water, for example, may enter into the movable space, and eventually adhere to the mass body. As a result, a displacement characteristic of the mass body is varied, possibly exerting influence on detection of acceleration.
An effective way to avoid generation of a crack is to provide a sufficient thickness of the cap or of the substrate of the semiconductor chip, thereby enhancing their rigidity. However, this method results in an increased size of device which is not desirable.